hdf5_msb

[svn-r15367] Description:
    Add check to avoid mounting the a file on a group twice, then the mounts
are done on the same HDF5 file, but opened with separate H5Fopen calls.

    Also add new 'mounted' flag to the H5G_info_t struct, queried with the
H5Gget_info() API call, to allow applications to detect and avoid this
situation.

    This probably fixes Bz#1070 also, I'll check with Dan Anov (who reported
a different sort of behavior, but seems to have the same underlying problem).

Tested on:
    Mac OS X/32 10.5.4 (amazon)
    Linux/64 2.6 (chicago)